A sliding member using a ceramic sintered body is being applied by utilizing its wear resistance to mechanical seal rings used in, for example, the mechanical seals of fluid equipments. The mechanical seal is aimed to completely seal fluid and is one of shaft sealing devices used in the rotating parts of various types of machines. The mechanical seal ring is made up of a rotary ring that slidingly contacts the rotating parts of the various types of machines and is movable in the axial direction in accordance with the wear of a sliding surface, and a stationary ring being immovable. The mechanical seal ring operates to restrict the fluid leakage at an end face substantially vertical to a relatively rotating shaft.
As a material constituting the mechanical seal ring, a carbon material, cemented carbide, a silicon carbide sintered body or an alumina sintered body is used mainly. In the recent years, the alumina sintered body or the silicon carbide sintered body which has high hardness and high corrosion resistance and also has a low coefficient of friction and excellent smoothness during sliding is often used.
Japanese Unexamined Patent Application Publication No. 7-33550 describes a mechanical seal member containing a silicon carbide sintered body having predetermined pores. This document also describes that these pores have a porosity of 2 to 12% by volume and have a spherical shape, and they are uniformly distributed into the sintered body and have a mean diameter (a mean pore diameter) of 50 to 500 μm.
Japanese Unexamined Patent Application Publication No. 2002-274953 describes a sliding body formed by a silicon carbide sintered material in which porous silicon carbide structures having fine pores are independently and dispersedly arranged in a dense silicon carbide structure, and a mechanical seal composed of the sliding body. FIG. 11 is a micrograph showing the pores existing in the sliding body. The portions looked like black in FIG. 11 are the pores. The coefficient of variation obtained from the average value of the circle equivalent diameters of portions where the pores appearing in the micrograph are opened (hereinafter referred to as opening portions), and the standard deviation thereof is approximately 0.79.
Japanese Unexamined Patent Application Publication No. 2002-147617 describes a mechanical seal that one or both of two seal rings rotating and slidingly contacting with each other are formed by a silicon carbide sintered material in which independent pores having a mean pore diameter of 10 to 40 μm are uniformly arranged, and the porosity thereof is 3 to 10%. FIG. 12 is a micrograph showing the pores existing in the seal rings constituting the mechanical seal. The portions looked like black in FIG. 12 are the pores. The coefficient of variation obtained from the average value of the circle equivalent diameter of opening portions of the pores appearing in the micrograph, and the standard deviation thereof is approximately 0.85.
Each of the above sliding members has a low coefficient of friction and has excellent smoothness during sliding. Hence, they may be suitably used for mechanical seal rings and other sliding members in a relatively short term.
However, the mechanical seal member described in Japanese Unexamined Patent Application Publication No. 7-33550 is low in the product (PV value) of a sliding surface pressure (P) and the circumferential speed (V) of the sliding surface, which is the index indicating the performance of a mechanical seal, and the mean pore diameter thereof is as large as 50 to 500 μm. Although needless to say when the pores having such a large mean pore diameter exist independently on the sliding surface, if the pores exist in mutual connection relationship on the sliding surfaces, it seems to be difficult to maintain sealing properties for a long term.
In the sliding body described in Japanese Unexamined Patent Application Publication No. 2002-274953, pre-granules obtained by granulating silicon carbide powder for a porous sintered body containing no sintering additives are granulated by merely mixing silicon carbide powder for a dense sintered body. The pores are therefore partially aggregated as apparent from FIG. 11, and it seems difficult to maintain sealing properties for a long term.
In the mechanical seal described in Japanese Unexamined Patent Application Publication No. 2002-147617, a pore forming agent, such as polystyrene, for forming pores is added to raw material powder. However, the pore forming agent is susceptible to aggregation. As apparent from FIG. 12, this increases the ratio of interconnected pores in which a plurality of pores are connected to each other to form a long slender shape having a large maximum diameter. Consequently, a long-term continuous use of the mechanical seal seems to deteriorate sealing properties because during sliding, stress concentrates at the peripheral portions of the contour forming the interconnected pores, thus being susceptible to degranulation.